Known industrial processes for producing acetic acid include a process which comprises allowing methanol to continuously react with carbon monoxide in a liquid phase having a low water content in the presence of a rhodium catalyst, a metal iodide, and methyl iodide to produce acetic acid with a high productivity. In the reaction liquid, there are small quantities of by-products (impurities), for example, a carbonyl compound (e.g., acetaldehyde, butylaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and an aldol condensate thereof) and an organic iodide (e.g., a C2-12alkyl iodide such as ethyl iodide, butyl iodide, or hexyl iodide). These impurities result in low quality of acetic acid. Extremely small quantities of reducing impurities present in acetic acid can be evaluated by a permanganate reducing substance test (permanganate time), and this evaluation method detects very small concentrations of impurities which are difficult to determine quantitatively even by a current advanced instrumental analysis. Moreover, some of these impurities have adverse effects in relation to use of acetic acid. For example, in a case where vinyl acetate is produced from ethylene and acetic acid, such impurities are known to deteriorate a palladium-series catalyst used. Unfortunately, a carbonyl compound such as acetaldehyde and a C1-12alkyl iodide cannot be removed sufficiently by an ordinary means such as distillation, due to having boiling points in close proximity.
In a continuous reaction process, removal of a carbonyl compound in a process recycle stream has been attempted. For example, Japanese Patent Application Laid-Open Publication No. 4-266843 (JP-4-266843A, Patent Document 1) discloses a method for removing carbonyl impurities, which comprises contacting a methyl iodide stream recycled to a carbonylation reactor with an amino compound for forming a water-soluble nitrogenous derivative by a reaction with carbonyl impurities; separating an organic methyl iodide phase from an aqueous derivative phase; and distilling the methyl iodide phase to remove the carbonyl impurities. However, the carbonyl impurity content in the organic stream recycled to the carbonylation reactor is still high, and it is difficult to sufficiently remove the carbonyl impurities. Moreover, the method described in this document requires removal of the nitrogenous compound.
Japanese Patent Application Laid-Open Publication No. 8-67650 (JP-8-67650A, Patent Document 2) discloses a process for producing a highly purified acetic acid, comprising the step of allowing methanol to continuously react with carbon monoxide in the presence of a rhodium catalyst, an iodide salt, and methyl iodide to produce acetic acid, wherein the concentration of acetaldehyde in the reaction liquid is maintained at not more than 400 ppm by removing acetaldehyde from the process liquid being recycled to a reactor. This document is attentive to that the impurities are formed mostly in the reaction system and that these impurities originate in by-product acetaldehyde formed in the reaction system. Thus, according to the document, the carbonyl compound content or the organic iodide content is reduced by controlling the concentration of acetaldehyde in the reaction system, and high-purity acetic acid is obtained.
Further, this document relates to a process for producing acetic acid while removing acetaldehyde and discloses a process which comprises separating a reaction liquid into a volatile phase containing acetic acid, methyl acetate and methyl iodide and a low-volatile phase containing a rhodium catalyst; distilling the volatile phase to form a product containing acetic acid and an overhead containing methyl acetate and methyl iodide; contacting the overhead rich in carbonyl impurity (particularly acetaldehyde) with water to form an organic phase containing methyl acetate and methyl iodide and an aqueous phase containing the carbonyl impurities, and recycling the organic phase to a reactor. Moreover, as a concrete method for separating methyl iodide from the concentrate containing the carbonyl impurity, this document discloses that it is preferred to selectively extract acetaldehyde by distilling and separating an acetaldehyde stream containing methyl iodide from the process stream and subjecting the resulting acetaldehyde-rich stream to extraction with water.
According to this document, acetic acid is purified by distilling the volatile phase (by a first distillation column) to form the overhead containing methyl acetate and methyl iodide (e.g., a lower phase formed by liquid-liquid separation of the overhead); distilling the overhead by a distillation column; and subjecting an acetaldehyde-concentrated stream from the top of the column to extraction with water for removing acetaldehyde. In the whole process, although acetaldehyde is concentrated in the overhead to some degree, the concentration of acetaldehyde in the overhead is not so high. Accordingly, acetaldehyde cannot be removed efficiently in a process for treating a liquid having a low concentration of acetaldehyde.
WO2010/053571 (Patent Document 3) discloses a process for producing acetic acid, which comprises carbonylating methanol, subjecting a reaction mixture to flash distillation, distilling the resulting low boiling point component by a first distillation column to form an acetic acid stream and an exhaust gas containing a light end fraction (e.g., methyl iodide) andanaldehyde impurity (e.g., acetaldehyde), washing the exhaust gas with an absorption solvent (such as acetic acid or methanol), stripping the light end fraction and the aldehyde impurity from the absorption solvent to form a light end stream, purifying the light end stream by a second distillation column to remove the aldehyde impurity, subjecting the distillate to extraction with water to remove the aldehyde impurity, and recycling a light end fraction purified from the light end stream into the reaction system. Unfortunately, since the absolute quantity of acetaldehyde contained in the exhaust gas is small, acetaldehyde cannot be removed efficiently.